This invention relates generally to corrosion-resistant, multiple-wall pipe structures and more particularly to a technique for preventing deterioration of the corrosion resistance of parts undergoing heat processing of corrosion-resistant, multiple-wall pipe structures for use as oil flowline pipes and the like.
The type of corrosion-resistant, multiple-wall pipe (hereinafter referred to as a double-wall pipe) with which this invention is concerned is that comprising an outer pipe and a corrosion-resistant inner pipe tightly fitted in the outer pipe by a method such as a thermo-hydraulic fit method, the inner pipe being made of a material such as austenitic stainless steel, an example of this type of double-wall pipes being disclosed in U.S. Pat. No. 4,332,073 to Yoshida et al.
As is known, pipes possessing corrosion resistance together with strength and heat resistance are used for oilfield pipes and the like. Principally, a metal such as carbon steel is being used for strength, while a metal such as stainless steel is being used for resistance to corrosion. However, for the purpose of satisfying a requirement for such performances, composite multiple-wall pipes or double-wall pipes formed by laminately joining pipes of these two metals are being used.
Piping requiring the above mentioned performances are assembled by using such double-wall pipes with appropriate couplings, joints, fittings, branch pipes, etc., of double-wall construction. For reasons such as good sealing, rapidity of work, and facility of the assembly operation, joints such as butt-welded joints are often used.
More specifically, in the case of double-wall pipes each comprising an outer pipe made of carbon steel and a stainless-steel lining fitted against the inner wall surface of the outer pipe, a plurality of these pipes are butt welded end-to-end to form a long double-wall pipe. However, because of the great quantity of heat input through this butt welding, problems such as corrosion of the pipes due to lowered corrosion resistance of the stainless-steel lining at the welded ends arises.
When heat is applied to the stainless-steel lining to carry out the butt welding, chromium carbide is precipitated in the lining, based on the same principle as the "sensitizing treatment", and inter-granular corrosion arises, while, as a result of the heat applied, residual tensile stress is created in the lining, causing stress corrosion cracking.
As a counter measure against the stress corrosion cracking, it has been a common practice to cool the stainless-steel lining internally while the butt welding is being carried out and/or to hydraulically expand the lining locally after the welding has been carried out, and by carrying out these procedures it has been possible to obviate the problem. However, there has been no counter measure for the intergranular corrosion because the solution treatment to cause the precipitated chromium carbide to disappear after the welding cannot be carried out for the reason that part of the lining, adjoining the lining part subjected to the solution treatment, is necessarily heated to a sensitizing temperature range in which chromium carbide is precipitated.